1. Field of the Invention
This invention relates to a semiconductor light emitting device, and more particularly to a semiconductor light emitting device which has a semiconductor light amplifier as a light source and controls the output wavelength to a desired value by returning a selected wavelength component of light emanating from the semiconductor light amplifier to the semiconductor light amplifier.
2. Description of the Related Art
There have been made various attempts to obtain a high intensity light beam of a single wavelength using a semiconductor. An example of such a semiconductor light emitting device is disclosed in "ELECTRONICS LETTERS" Vol. 29, No. 14, (1993), pp. 1254.about.1255.
As shown in FIG. 12, the semiconductor light emitting device has a semiconductor light amplifier 1 as a light source. Light emanating from the rear end face 1a of the semiconductor light amplifier 1 is collimated by a lens 2 and then caused to impinge upon a reflection diffraction grating 3. With this arrangement, light component 4 having a wavelength selected by the diffraction grating 3 is returned to the semiconductor light amplifier 1 and accordingly the wavelength of light 4F emanating from the front end face 1b of the semiconductor light amplifier 1 is locked to a single wavelength, whereby a high quality and high intensity light beam of not less than 1.5 W near the diffraction limit can be obtained.
Further in the semiconductor light emitting device, the output wavelength can be changed within a certain range by changing the angle of the diffraction grating 3 so that the angle of incidence at the diffraction grating 3 changes.
In the conventional semiconductor light emitting device where the output wavelength is selected by use of an external optical system, the phenomenon that the angle of diffraction at the diffraction grating changes with wavelength is utilized and accordingly a spatial aperture for cutting light beams which travel deviating from the predetermined optical path after reflection at the diffraction grating is required for selection of the wavelength. For this purpose, as the semiconductor light amplifier 1, a tapered stripe amplifier where the stripe width Wi on the side of the external optical system is narrowed, for instance, to 4 .mu.m has been conventionally used as shown in FIG. 12 and the narrow stripe is used as an effective spatial aperture. When the stripe Wi is narrow, the transverse mode becomes optically single and since the stable mode is coupled with the diffraction grating, the high output light beam amplified by the tapered stripe amplifier can be of a high quality close to the diffraction limit.
However such a limit in the stripe width makes it difficult to meet a high output requirement.
Further in the conventional semiconductor light emitting device, the angle of the diffraction grating must be set with a high accuracy in order to obtain a desired output wavelength, which is very troublesome and difficult. As the numerical aperture NA of the lens 2 is increased in order to increase the coupling efficiency of the light component emanating from the semiconductor light amplifier 1 with the diffraction grating 3, the depth of focus reduces and a higher accuracy is required in alignment of the optical system. Accordingly shift in relative positions between the optical parts due to change with time or vibration can results in deterioration in performance and/or instability of light output.
Further the conventional semiconductor light amplifier is also disadvantageous in that the oscillation mode becomes instable due to coupling of a plurality of transverse mode outputs if the stripe width Wi on the side of the diffraction grating is not sufficiently narrow.
Further the diffraction grating is generally very expensive and it has been difficult to manufacture the conventional semiconductor light emitting device at a low cost.